If an individual has editing rights to a video then they can record an audio description file and add it as an extra track to the video using editing software.
Transcript for Video: Adding Audio Descriptions Using Video Editing Software (2:41)
Using video editing software to add an extra track to a video you do not own may be a copyright violation. In this situation, there are some free tools available for adding audio descriptions to online videos.
YouDescribe is a free online tool that allows users to record audio descriptions for YouTube videos from their web browser. The user does not have to download or change the original video.